A particle board is formed by compressing a layer of particles, generally a mixture of cellulosic or other particles, with a binder in a platen or other press at a temperature sufficient to activate the binder. The particles normally consist of wood chips, splinters, sawdust, and fibers and the majority of the particles are normally elongated. The binder is normally a thermally activatable synthetic resin which is mixed with the particles, or it is a resin which is intrinsically present in the wood. When a layer or mat of such particles is subject to heat and compression the binder is activated to bind the particles into a coherent sheet known in the art as a particle board.
It is well known from the commonly owned earlier U.S. Pat. Nos. 4,063,858 and 4,068,991 of Dec. 20, 1977 and Jan. 17, 1978, respectively, to form the mat for making such a particle board by dispensing the particulate material onto a conveyor by means of an array of so-called disk rollers. These disk rollers each comprise a plurality of parallel disks rotatable about a common axis, with the disks inter-digitated and all of the axes lying substantially in a common plane. The disks are all rotated in the same direction so that their upper portions move in a horizontal transport direction which is identical to the transport direction of the upper reach of a conveyor belt or other surface extending along underneath and parallel to the array of disk rollers.
Wood particles are fed onto the downstream end of this array of disk rollers which form vertically throughgoing spaces that increase in cross-sectional area from the downstream portion to the upstream portion of the array, normally by providing fewer disks on the upstream rollers than on the downstream rollers. The material is thus classified, with the smaller particles falling through the downstream portion and the larger particles falling through the upstream portion of the array. A particulate mat is therefore formed underneath this array which has an upper mat surface that forms with the conveyor surface an acute deposition angle open in the downstream direction. In the above-discussed system the array of rollers is perfectly parallel to the conveyor surface and situated well above the conveyor surface so that, although the particles are classified with the smaller particles falling through the array at the upstream end and the larger particles falling through at the downstream end, the direction of elongation of the particles in the mat is virtually random.
It is known also from U.S. Pat. No. 3,115,431 issued Dec. 24, 1963 to STOKES and YAN to closely juxtapose the array with the conveyor surface, and indeed to orient the plane of the axes of the disk rollers perfectly parallel to the upper mat surface underneath the array. In this manner it is possible to deposit the particles so that they lie principally parallel to the transport direction. Such a system has been found extremely useful in that it produces a board which has extremely good strength in at least this one direction. What is more, it makes an extremely compact board.
The boards made with particles arranged generally randomly are advantageous in that they have approximately the same strength in all directions. The boards made of particles which are aligned parallel to each other have excellent strength in one direction, but are relatively weak in another. Furthermore these boards with parallel particles are frequently so very dense that it is difficult to nail through them, and they are likely to split. Nonetheless, the parallel-fiber boards have a surface appearance almost resembling that of natural wood.